doublte walled flues

I'm reading another thread about furnaces, the long thread where the
house won't heat up "furnace blowing all the heat up the chimney" and
someone wrote that flues are double-walled. Is that always the case?
Maybe his furnace is gas and mine is oil? Because my flue is single-
walled. Is that bad? As long as it doesn't cool off so much that the
chimney doesn't draw well, any heat lost from the non-insulated flue
would heat the basement.
When I get a new oil furnace (no gas supply) should the new one have a
double-walled flue?

Okay, I get it. It also makes me feel better about last weeks
experience in Home Depot. I was in a hurry so I asked a clerk where
flue pipe was, and he said something about double wall and said they
didn't have any. Later I asked another clerk and he walked me right
to what I needed. The first clerk wasn't young but he was living in
the present. The second guy was the same age and found it for me, but
the first had a reason for his mistake.
Thahks a lot.

There is virtually no oil fired equipment in my area, so I all my
experience has been on gas units. The main thing about double walled "B"
vent is allowable clearance to combustibles. It is generally 1 inch with
double and 6 with single. With single wall, a 4 inch pipe would need to
have a 16 inch diameter hole in a roof. Larry

Yes, after about 8 feet of single wall, winding up and around the
furnace to the the metal pipe chimney, and far from combustibles, the
chimney is much bigger in diameter and I think it is double wall, but
I forget why I think that. Probalby from looking at it from the roof
end.
Thanks to everyone.
There is gas 400 feet from my house, but my n'hood of 100 townhouses
has no gas. At least I know my neighbors won't burn my house down
with a gas explosion, but that's the only benefit.
The electric stove broiler isn't hot enough either afaic.

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